Don battling dry air and wind shear

Tropical Storm Don continues to be an unimpressive low-end tropical storm as it continues northwest towards the Texas coast. Don formed yesterday afternoon from an African tropical wave that moved into the Gulf of Mexico under a region of low wind shear. Don's formation date of July 27 is nearly a month ahead of the usual August 23 date for the arrival of the season's fourth named storm of the year. There is currently no hurricane hunter airplane in Don, and a new airplane is not due in the storm until tonight. The last center fix at 1pm EDT found surface winds of 45 mph and a central pressure of 1005 mb, a 4 mb rise from earlier this morning. Water vapor satellite images show a region of dry air to the northwest of Don, over the western Gulf of Mexico. Wind shear as diagnosed by the University of Wisconsin CIMSS group and the SHIPS model show a moderate 5 - 15 knots of shear from strong upper level winds out of the north. This shear is creating problems for Don by injecting dry air into the system. Visible satellite imagery from early this afternoon showed the presence of surface arc-shaped clouds expanding outwards to the north from the center of Don. These type of clouds are a sign that the storm is struggling with dry air. When dry air at middle levels of the atmosphere gets injected into thunderstorms due to wind shear, the dry air tends to create strong downdrafts that rob the storm of moisture. These downdrafts spread out at the ocean surface and create arc-shaped surface cumulus clouds.

Figure 2. The latest drought map for Texas shows that over 75% of the state is in exceptional drought--the highest category of drought. Image credit: U.S. Drought Monitor.

Forecast for DonThe big question for Don is, will it bring significant rains to Texas? According to the National Climatic Data Center, the six-month period ending in June 2011 was the driest on record. Average rain between January and June was more than eight inches (203 millimeters) below average in Texas, and the state experienced record heat between April and June. The heat and lack of rain have brought exceptional drought--the highest category of drought--to over 75% of the state. Don has the potential to bring some decent drought-busting rains to the state. If Don can expand in size and intensify to a 50 - 55 mph tropical storm, it has the capability to bring hundreds of millions of dollars worth of beneficial rains to the state. We don't want Don to stay in its current state, which is too small and weak to bring significant rains to Texas. If Don follows the current NHC forecast, which brings the storm up to a moderate-strength tropical storm, that would be just right. Don's small size makes it prone to dry air and wind shear, though, and it is uncertain whether the storm can overcome these problems enough to become a significant rain maker. NHC gave Don a 12% chance of intensifying into a hurricane in the 11am advisory, which is a reasonable forecast, since Don is running out of time to get its act together in time to become a hurricane. None of the computer models is predicting Don will become a hurricane.

For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast. The 11 am version of this forecast shows that Port O'Connor, Texas has the highest chance of tropical storm-force winds (39+ mph): 45%.

New hurricane archive search featureThe autocomplete entities in the wunderground search box has been extended to include hurricanes, so you can now search for a storm by name, year, or basin. Here are some examples in case you feel like exploring your new options:

Yesterday I posted"Don will go down as one of the MOST beneficial TS since records were kept. He will make landfall with a paltry windspeed of 50-55 mph. There will be sustained tropical rains and mild breezes that will put a massive dent in the Texas and maybe even New Mexico droughts. There will be no loss of life and minimal property damage. Maybe a tree limb or 2 will fall, but even this could be a good thing because tree limbs have to go sooner or later. If my vision works out, thank you Don."Looking like a home run so far!

Quoting Levi32:Checking in for break. Can't wait to dive into data from multiple planes when I get home.

Levi your back! how long can you stay on the blog and the 18z gfs brings that central atlantic tropical wave as a 1007mb low into the carribean further south.. and also don looks to be stregnthining.. any thoughts?

hey Sheri...good as to be expected...still think Don is going to go outside the models and get stronger than they think...also think it wont be as far south as they think...the high that was supposed to guide it moved faster than expected (we were supposed to get rain on the SC coast till friday and it lifted out before 5pm yesterday)...

See you were right with your "gut" feeling. We should never ignore them.

Austin area not expecting much from Don but may not hit 100 Saturday, forecast is only 98.

TROPICAL STORM DON continues on a NW track. We expect landfall late Friday night or early Saturday AM along the lower coast of Texas. Slight strengthening is possible before landfall, but it will likely stay below hurricane strength. Computer models Thursday afternoon have shifted the forecast track a bit farther south, which is not good news for our rain chances. Once Don arrives, it will not linger. By Sunday it will already be out in West TX with our rain chances quickly coming to an end.

In the short term, a few spotty showers appeared on radar Thursday, but most areas missed out. Mostly clear skies will turn mostly cloudy by Friday morning with lows in the upper 70s. We'll call for isolated showers and storms (20%) on Friday as Gulf moisture continues to pour in to TX ahead of Don. It will still be hot with temps in the upper 90s to near 100, and feeling hotter with the humidity. The slight rain chance continues Friday night,.

Once Don makes landfall Saturday, the heaviest rain will likely be south of our area. Parts of South Texas may see 3-5" or more. Latest projections indicate the potential for a half inch or less near Austin, with heavier totals southward, and less northward. We're calling for a 30% chance in Austin.

I think it will. We may see one more lull and see the current activity pushed southward but if so I think another burst would take over later.

As of now 90% or more of Don's convection is south of the surface center.

I'm interested to compare the 5pm position/satellite (I did so in a post further back) to an 8pm counterpart.

At 5pm the center was at or barely under the nothernmost fringe of the large convective mass.

While there is argument over whether or not the CIMSS shear values are correct or not - what is obvious is strong northerly winds continue to drive Don's convection south of the center.

I want to take a look at continuity between 5pm and 8pm and see if he's improved this situation and by how much.

Too many folks looking at the large blowup of convection and aren't noting where it is in relation to the center - giving a false sense that it's a well organized and intensifying system - when it's clearly not as of 5pm.

===============This was as of 5pm, et, along with the center fix placed over the visible imagery:

==============While the image is 45 minutes older than the center fix this still illustrates just fine what poor shape Don is in. CIMSS shear map is obviously far from reality. There is a LOT of shear devastating Don.

Recon has confirmed that the surface center is south of there and more in line with the convection.

hey Sheri...good as to be expected...still think Don is going to go outside the models and get stronger than they think...also think it wont be as far south as they think...the high that was supposed to guide it moved faster than expected (we were supposed to get rain on the SC coast till friday and it lifted out before 5pm yesterday)...

Tig thanks for the answer. I've asked different questions to folks on her but since I'm a nobody I guess they don't need answering. Have a good night.

Gosh, I love rapid scan. Anyway, Don appears to be producing enough heat now to fight off the shear effects. In other words, he is starting to produce his own anticyclone in the upper levels. That is why we are seeing such an improvement in the outflow:

Heya Tig, ya doing good? what u think about Don and the other wave out there. I guess Flood disappeared I said hello and usually he always talks back.

sheri

hey Sheri...good as to be expected...still think Don is going to go outside the models and get stronger than they think...also think it wont be as far south as they think...the high that was supposed to guide it moved faster than expected (we were supposed to get rain on the SC coast till friday and it lifted out before 5pm yesterday)...

i say San Diego California,Don gonna live across 1500 miles of desert and mountains, and emerge into the pacific ocean, become a Cat 1 Hurricane in 70 degree water and slam the Hotel Cel on Coronado Island CA.

Just when I got into the conversation about the wave seems like others re bringing it up.I'm not to faithful in the models right now.Seeing that they've forecasted several storms to from off of Afica.And none have occured.Maybe because they were early in the season.